This is like being in a really big, long mine shaft. It's really just carved right into the earth.

The tunnel is absolutely amazing. It is probably the biggest tunnel on the southern border so far.

They could have been digging this for years, just looking for a place to come up.

It was like being in a cavern or a cave.

It's too early for us to tell whether this one is connected to any previous tunnel.

They didn't know what the air quality or structural integrity might be. Agents had to wade through chest-high water. The first time through was pretty scary.

This one is just absolutely amazing. It's just huge. It really looks like a professionally done mine shaft.

We're very concerned. When we find these tunnels, we see that as a vulnerability to our national security.

What is key to the investigation is how long the U.S. exit has existed.

We're chasing down investigative leads throughout San Diego and Southern California. At the same time, we're working cooperatively with DEA and ICE (Immigration and Customs Enforcement) and Mexican authorities in Mexico in the Tijuana area. They're also running down leads.